A typical absorber for CO2 is a tower where exhaust gas from which CO2 is to be absorbed/removed, is brought in counter current contact with an aqueous CO2 absorbent, such as an amine, in a contact zone. The contact zone normally comprises a CO2 absorption section to increase the contact surface between the absorbent and the gas.
Absorbent having absorbed CO2 is collected below the contact zone, and exhaust gas reduced in CO2 content is released from the top of the absorber.
An amine based CO2 absorber is normally equipped with one or more water wash section(s) above the CO2-absorbing contact zone. The main objective of the water wash is to absorb amine vapour in order to minimise emission of amine to air. Another objective is to cool the gas and condense water in order to fulfil the requirement for water balance across the entire absorber. The condensation of water from the flue gas is a source for wash water makeup. Excess water in the water wash system is bled off and routed to the amine section below, and the effect of these make-up and bleed streams is reduced amine content in the water wash liquid.
One normally assumes close approach to equilibrium in the top of the water wash packed section with liquid recycling, meaning the gas will contain an amine partial pressure equivalent to the liquid amine vapour pressure, which again is dictated by the liquid temperature, amine concentration, CO2 loading and pH. Hence a water wash operating at low temperature with high liquid replacement rate is preferred in order to minimise the amine slip to atmosphere, since lower temperature and lower amine concentration decreases amine vapour pressures.
EP 0502596 A (MITSUBISHI JUKOGYO KABUSHIKI) 09.09.1992 relates to an absorber for a CO2 capture plant where the CO2 lean exhaust gas is washed and cooled, and the temperature of the gas leaving the absorber is substantially equal to the gas introduced into the absorber. The washing is done to reduce amine emission. The temperature is controlled to avoid loss or building up of water in the plant.
US 2003045756 A (MIMURA TOMIO) 06.03.2003 relates to an absorber in a CO2 capture plant wherein demisters are provided between absorption and washing sections and between washing sections to reduce amine emission. Additionally, amine vapour is removed from the gas by washing and cooling the gas.
Extensive emission measurements have been performed by the applicant on an operating pilot, treating flue gas from a coal fired boiler. Various CO2 absorbing solvents have been tested, including 30wt % MEA. The pilot contains two water wash sections, where the upper section can be utilised as an acid wash in dedicated campaigns. An acid wash, as described in EP 2335802 A (MITUBISHI HEAVY INDUSTRIES, LTD.) 22.06.2011 and WO 2010/102877 A (AKER CLEAN CARBON AS) 16.09.2010 has proven effective at capturing gaseous phase alkaline compounds. FIG. 1 (Prior art) illustrates a simplified scheme of the pilot. Both online analysis (FTIR) and offline sampling of gas have been conducted.
There has surprisingly, under certain periods, been detected high emission of amine, even with operation of the acid wash in the absorber top that eliminates emission of ammonia and volatile alkyl amines. The amine emission could not be explained by the water wash vapour-liquid equilibrium assumption. It is discovered that this emission is in the form of mist, and that the more hydrophilic compounds such as MEA tends to be highly accumulated in the mist. The FTIR gas analyser samples and evaporates the mist in the heated sampling line (working at 180° C.), and hence measures the total amine content as vapour. It has been discovered that the mist precursors are ultrafine solids particles of fly ash, soot or salts in the flue gas entering the absorber.
Another test was performed at a smaller scale pilot, where the flue gas source is a propane burner. During normal operation, the emission from the pilot was not detectable by the FTIR. The burner was then manipulated to burn with excess fuel, resulting in high CO and soot formation. High amine emission and visible mist plume was observed, indicating soot particles acting as mist nucleation seeds.
These fine particles are initially acting as nucleates for water condensation in regions inside the absorber where water saturated gas is rapidly cooled. When formed, these mist droplets absorb amine from the surrounding gas phase. Emission measurements during campaigns with amine solvents such as MEA have proven that, in the absorber system with two water wash sections, the mist borne emission is the main contributor to overall amine emission. On the other hand, the emission of ammonia (which is a degradation product of amines such as MEA) is not related to the mist emission. This is ascribed the limited solubility of ammonia and hence limited accumulation in the mist droplets.
Mist with small droplet size, when formed, is very difficult to remove in wet scrubbers and conventional demisters. The mist droplets are in the size range of 0.1-10 μm diameter, and the mist forms a visible white plume from the top of the pilot absorber.
Mist elimination by fibre type demisters is known from other industries. The superficial velocity through such demisters has to be very low and the pressure drop is high, making this type of demisters less attractable for large gas volume applications like power plant flue gas cleaning. Wet electrostatic precipitator (ESP) is also proven efficient for removing mist and fine dust, but has high investment and operating cost.
The applicant has experienced that formation of mist may also be reduced by removing or reducing the content of fine and ultrafine particles from the incoming gas before introduction of exhaust gas into the CO2 absorber. Conventional processes for removal of fine and superfine particles, such as ESP and wet flue gas desulphurization (FGD) have limited efficiency in capturing particles in the sub-micron range. As for removal or reduction of mist, wet ESP may be used to reduce or remove the sub-micron particles. As mentioned above, the investment and operating cost for wet ESP are high.
The solutions according to prior art is that they are not efficient in reducing mist following the gas that is washed.
The problem to be solved by the present invention is to eliminate or substantially reduce the release of amines or degradation products thereof from an amine based carbon capture plant. More specifically, the invention is directed to reduction of the release of amines caused by formation of mist that is released to the surroundings together with the CO2 lean exhaust gas.
Accordingly, alternative solutions allowing reduction of the emission of mist, or droplets, and most specifically mist containing high concentration of amines or degradation products thereof, from the absorber of an amine based plant for CO2 capture is therefore sought.